I've mentioned this before, but now I've settled into Usenapp as my primary newsreader I wanted to make sure it got its own post here (because I keep trying to do it!).
I read using space/n set to "jump to next unread", which means if I do jump to next unread I often have no idea where the post I just came from was. Sometimes I'd like to go back and reply, but I don't realise until several articles later. Sometimes I've just accidentally double-tapped n.
My old newsreader Forte Agent used backspace to step back through the previous articles you'd seen (and shift-backspace to step forward through them again), so I could just backspace a few times and then reply. No other effects - it doesn't toggle unread status or anything - just tracks back through the viewed posts. Trackback history lost on app exit, no problem there. History list 25 deep would be perfectly sufficient, I imagine.
Backspace is already committed so I'm not sure what'd be best, maybe cmd-pageup/pagedown but I've got a wide keyboard which is rare in Mac land! Cmd-[ and ] perhaps. Do other Mac newsreaders have this function already set on certain keys?